Water softening systems are used in households and by industry to replace hardness cations such as calcium and magnesium with sodium ions by passing an incoming water supply through a bed of cationic exchange resin beads or particles. When the ion exchange resin bed periodically becomes saturated with ions removed from the incoming water and depleted of sodium ions, it is recharged by passing a brine solution consisting essentially of sodium chloride through the resin bed. This replenishes the bed with sodium ions and removes the calcium, magnesium, or other ions previously removed from the incoming water.
The iron problem is well known in the softening art, and attempts have been made to remove iron in all its forms from water softening resin beds. The exchange capacity of an ion exchange resin bed deteriorates as the iron in the incoming water collects in the resin bed and is not removed by the recharging process. Sooner or later, depending on the level of maintenance of the resin bed and the characteristics of the water supply being softened, the resin becomes "fouled", meaning that the resin bed's capacity to soften water has diminished so much that the resin must be specially treated to restore its softening capacity or be replaced.
Iron can exist in the water supply as clear water iron, red water iron, bacterial iron, colloidal iron, or tannate iron. Clear water iron is iron in the divalent (ferrous), soluble state. Clear water iron is not visible when the water is drawn, but when the water is allowed to stand for a prolonged period the ferrous ions are oxidized by air to become ferric or trivalent ions, which settle as a precipitate of ferric hydroxide. The iron may also oxidize after having been exchanged into a resin bed, which may prevent it from being removed by regeneration. Red water iron is already oxidized to the ferric state when it reaches the water softener. Water containing red water iron is cloudy and orange when drawn. This form of iron may be filtered by the resin bed or may be passed and be present in the softened water. Bacterial iron is a third troublesome form of iron, and is caused by iron crenothrix bacteria which feed on the iron in the water supply. These bacteria thrive in water softener resin beds supplied with ample iron, and the resulting biomass clouds the water system, creates a bad taste and odor in the softened water, and occasionally releases large, unsightly masses of rust colored material. Colloidal iron is similar to red water iron, but is composed of particles too small to settle. Colloidal iron will normally pass directly through a water softener. Finally, tannate iron, which is quite similar in appearance to colloidal iron, is ferric iron complexed and held in solution by tannates or other naturally occurring soil ingredients. This final form of iron usually passes through a water softening resin bed. Iron present in any of the previously discussed forms can foul the resin bed. Oxidation of ferrous iron captured by the resin beads can crack them, thereby physically degrading the resin bed as well.
Chelating compounds for sequestering iron, including citric acid, are taught in U.S. Pat. No. 3,454,503. A method for regenerating cation exchange resins fouled by iron by adding to the brine regeneration medium any of a variety of organic acids, particularly citric acid, is taught in U.S. Pat. No. 2,769,787. Citric acid is used commercially in water softening salt compositions to remove iron from the system. Compositions for regenerating resin beds, comprising a major proportion of an alkali metal chloride, an alkali metal carbonate, and an alkali metal carboxylate chelating agent such as sodium and potassium citrate are disclosed in U.S. Pat. Nos. 4,071,446; 4,083,782; and 4,116,860. These compositions have extremely high concentrations of sodium citrate (5 to 15 per cent) and pH values of 7-9.5 and are not designed for regenerating conventional sulfonate-based resins.
U.S. Pat. No. 4,540,715 teaches that regular use of a synergistic combination of sodium citrate, as a sequestering agent, and a surfactant identified as an alkylated diphenyl oxide disulfonate can protect cation exchange beds against deterioration caused by the accumulation of iron, other insolubles, oily and fatty deposits, and other impurities found in various water supplies. Sodium citrate, the common name for trisodium citrate dihydrate, imparts a pH of about 8 to a brine containing it. Citrate ion, the active part of the sequestering agent, constitutes only about 64% of its weight because of the high sodium and water content.